## **Core Concept**
The Lac operon is a genetic regulatory system in *E. coli* that controls the expression of genes involved in lactose metabolism. It is a classic example of an operon, a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter. The Lac operon consists of a promoter, an operator, and three structural genes.

## **Why the Correct Answer is Right**
The Lac operon contains three structural genes: *lacZ*, *lacY*, and *lacA*.
- *lacZ* encodes for β-galactosidase, an enzyme that breaks down lactose into glucose and galactose.
- *lacY* encodes for lactose permease, which transports lactose into the cell.
- *lacA* encodes for thiogalactoside transacetylase, an enzyme with less clear physiological significance but thought to be involved in protecting the cell from toxic thiogalactosides.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests one structural gene, which undercounts the actual number of structural genes in the Lac operon.
- **Option B:** This option suggests two structural genes, which still undercounts the actual number of structural genes in the Lac operon.
- **Option D:** This option suggests more than three structural genes, which overcounts the actual number of structural genes in the Lac operon.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Lac operon's structure and function illustrate a critical concept in molecular biology: the regulation of gene expression in prokaryotes. Understanding the Lac operon helps in grasping how bacteria adapt to different carbon sources, such as lactose.

## **Correct Answer:** . 3.